Installling shower base--frustrations!

I am finishing up a bathroom-finally getting to a 3ft square basement shower that was cracked at the bottom. After removing the base and buying a base to fit the shower, on pre installation I came to the grim reality that the drain hole and 2'' pipe drain in the concrete did not line up, in fact two inches off. Speaking with a local plumber I was told to line up the drains at all cost. That I did and installed the gasket drain and sealed it to the pipe.

Three sides fit perfectly, but now I'm stuck with a 2'' gap that's seven inches deep (opposite the entering side) that I have filled with 2 2x4s for the seven inches. The big question: What can you cover this 36" x 2'' gap with to completely seal and waterproof the opening? Plastic, more fiberglass, sealant? (I don't want to spend another $100 bucks to buy another base)

The better answer is to move the drain line to line up. Or, you could make a traditional shower pan and tile it. I'm not a pro, but I can't think of a good way to fill thatopening and keep it waterproof, at least without spending a bunch. How's your fiberglass forming skils? You'd never get the finish to look like the existing walls, but you could bond that in. Very much of a hack, though. A picture would help. What exactly are the walls made of and how do they connect with the base?